For obtaining a radiation image, there has been recently proposed and practically used a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for example, in U.S. Pat. No. 4,239,968. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. After the reading procedure, a radiation image remaining in the radiation image storage panel is erased from the panel, and the panel is stored for the next radiographic process.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resources and economic efficiency because the radiation image storage panel can be repeatedly used in the method, while the radiographic film is consumed for each radiographic process in the conventional radiography.
In the method, as described above, a radiation image can be obtained with a sufficient amount of information by applying a radiation to an object at a small dose, so that this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The stimulable phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with stimulating rays such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described above, and it is desired for the radiation image storage panel used in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as for a radiographic intensifying screen used in the conventional radiography. The radiation image storage panel is used repeatedly as mentioned before, so that the panel is also desired to be resistant to physical shock from the viewpoints of reliability of the resulting image data, economical efficiency and easy handling.
For enhancing the sensitivity of the radiation image storage panel, there has been known the art that a light-reflecting layer is provided between the support and the stimulable phosphor layer by depositing a metal such as aluminum on a surface of the support, laminating a metal foil such as an aluminum foil thereon, or applying a coating dispersion comprising a binder and a light-reflecting material thereonto. As for the light-reflecting material, titanium dioxide, white lead, zinc sulfide, aluminum oxide, magnesium oxide and alkaline earth metal fluorohalides are employed as described in U.S. Pat. Nos. 4,380,702 and 4,621,196. A light emitted by the stimulable phosphor in the phosphor layer and advancing towards the support is reflected by said layer and released from the phosphor layer-side surface of the panel. Accordingly, the light advancing towards the support is also detected to further increase the sensitivity of the panel.
However, there is a problem that air bubbles are likely produced on the interface between the light-reflecting layer and the stimulable phosphor layer in the course of forming them by coating the support successively with a coating dispersion comprising a binder and a light-reflecting material and a coating dispersion comprising a binder and a stimulable phosphor (i.e., successive coating method), and the bubbles affect a resulting image to cause lowering of image quality (unevenness of image density). It is assumed that bubbles are produced by the fact that a solvent in a coating dispersion for the formation of a phosphor layer permeates the light-reflecting layer in the course of coating the dispersion on the previously formed light-reflecting layer, and air dispersively contained in the light-reflecting layer is raised and forms the bubbles on the surface of the light-reflecting layer.
For solving the above problem, U.S. Pat. No. 4,791, 196 proposes a process wherein a dispersion of a stimulable phosphor in a binder solution (i.e., coating solution for the formation of a stimulable phosphor) and a dispersion of a light-reflecting material in a binder solution (i.e., coating solution for the formation of a light-reflecting layer) are coated over a support in a superposed form, to form a stimulable phosphor layer and a light-reflecting layer simultaneously on the support. That is, the process utilizes a two-layer simultaneous superposition coating method. According to the two-layer simultaneous superposition coating method, a radiation image storage panel can be prepared by a simple process, and further a stimulable phosphor layer having a light-reflecting layer of high efficiency can be formed.
However, the radiation image storage panel is also de sired to be resistant to physical shock as well as to have a high sensitivity and give an image of high quality, as described hereinbefore.
The stimulable phosphor layer can be protected from chemical deterioration and physical shock by providing a transparent protective film as described above, but it is required to have increased adhesiveness (bonding strength) between the layers of the panel in order to prevent interfacial separation between these layers caused by the physical shock.
The radiation image storage panel prepared using the above-mentioned two-layer simultaneous superposition coating method is improved in the adhesion between the stimulable phosphor layer and the light-reflecting layer to a certain level, but the adhesion therebetween is required to be further increased in consideration of service conditions of the radiation image storage panel.